1. Field of the Invention
The present invention relates to a digital protective relay apparatus which realizes an accident determination section and a protection sequence section by an arithmetic operation of a digital processing unit such as a microprocessor.
2. Description of the Related Art
A malfunction of a digital protective relay apparatus for protecting a power system has a large influence on the power system and causes system down in the worst case. Therefore, a protective relay apparatus is required to have very high reliability.
The internal arrangement of a conventional digital protective relay apparatus is separated into a main detecting unit and a fail-safe unit. In most conventional apparatuses of this type, the main detecting unit and the fail-safe unit have each an accident determination section and a protection sequence section. A protective relay apparatus of this type is designed to output a trip command to a circuit breaker if trip outputs are generated by the protection sequence sections of the main detecting unit and the fail-safe unit.
FIG. 13 shows a practical arrangement of the conventional digital protective relay apparatus as described above. Referring to FIG. 13, reference numeral 1 denotes an accident determination section of a main detecting unit, and reference numeral 2 denotes its protection sequence section. The protection sequence section 2 receives an accident determination signal S.sub.RY obtained by the accident determination section 1, determines necessity of a trip output by predetermined sequence processing, and outputs a trip command as needed. Reference numeral 4 denotes a contact driver which receives a result signal indicating a logic operation result of the protection sequence section 2 and operates a contact circuit 6 in accordance with the result signal; 7, a fail-safe unit including an accident determination section and a protection sequence section for fail safe; and 8, a contact driver which receives an accident determination signal obtained by the accident determination section in the fail-safe unit 7 and operates a contact circuit 9 connected in series with the contact circuit 6 in accordance with the determination signal. When both of the contact circuits 6 and 9 are closed, a trip command Scb is output to a circuit breaker (not shown) provided in a power system.
Assume that in the digital protective relay apparatus having the above arrangement, although no accident occurs in a power system, the accident determination section 1 supplies the signal S.sub.RY indicating occurrence of an accident to the protection sequence section 2 due to a failure in the accident determination section 1. Therefore, a false operation command is supplied to the contact circuit 6, though the protection sequence section 2 and the contact driver 4 are normal. As a result, the contact circuit 6 is closed.
Since, however, no accident has actually occurred in the power system, the fail-safe unit 7 does not operate. Therefore, the contact circuit 9 is not closed because no operation signal is supplied to the contact driver 8.
According to the arrangement shown in FIG. 13, therefore, even if the contact circuit 6 is erroneously closed due to a failure in the accident determination section 1, a false trip command Scb is not supplied to the circuit breaker because the contact circuit 9 is not closed.
Similarly, even when the protection sequence section 2 or the contact driver 4 fails, a false trip command Scb is not supplied to the circuit breaker (not shown) if the fail-safe unit 7, the contact driver 8, and the contact circuit 9 have not failed.
Even when any of the fail-safe unit 7, the contact driver 8, and the contact circuit 9 fails and the contact circuit 9 is closed, since the contact circuit 6 is not closed if none of the accident determination section 1, the protection sequence section 2, the contact driver 4, and the contact circuit 6 have failed, a false trip command Scb is not supplied to the circuit breaker.
The above digital protective relay apparatus, however, must comprise the fail-safe unit having the accident determination section and the protection sequence section in order to increase its reliability. Therefore, the scale of the apparatus is increased to result in an expensive apparatus.
On the other hand, a digital protective relay apparatus constituted by only a main detecting unit is also available.
FIG. 14 shows a practical arrangement of a digital protective relay apparatus of this type. The apparatus shown in FIG. 14 is the same as that shown in FIG. 13 in arrangements of an accident determination section 1, a protection sequence section 2, a contact driver 4, and a contact circuit 6 but no fail-safe unit is used in the apparatus shown in FIG. 14.
In this protective relay apparatus, if one of the accident determination section 1, the protection sequence section 2, and the contact driver 4 fails, a false operation command is output to the contact circuit 6. In addition, since no section corresponding to the fail-safe unit shown in FIG. 13 is present, a false trip command Scb is directly output to a circuit breaker if the contact circuit 6 is closed. As described above, the false trip command Scb has an influence on a power system and causes system down in the worst case.